Means for forming grinding wheels for use in grinding internal screw threads



i 1941- J. E. WAKNWRIGHT 2.237.974

MEANS FOR FORMING GRINDING WHEELS FOR USE IN GRINDING INTERNAL SCREW THREADS Filed Nov. 14, 1959 s Sheets-Sheet 1 FIG. I.

April 8, 1941. J w mw g 2,237,974

MEANS FOR FORMING GRINDING WHEELS FOR USE IN GRINDING INTERNAL SCREW THREADS Filed Nov. 14, 1939 5 Sheets-Sheet 2 April 8, 1941- J. E. WAINWRIGHT 2,237,974

MEANS FOR FORMING GRINDING WHEELS USE IN GRINDING INTERNAL SCREW THRE 5 Filed NOV. 14, 1939 3 Sheets-Sheet 3 Patented Apr. 8, 1941 FOR USE IN GRINDING INTERNAL THREADS SCREW John Ernest Wainwright; Coventry, England, as-

signor to Coventry Gauge & Tool Company Limited, Coventry, England, a company of Great Britain Application November 14, 1939, Serial No. 304,431 In Great Britain November 8, 1938 4 Claims.

Thisinvention relates to an improved method of and means for forming grinding wheels and more particularly to the application of the crushing method for forming grinding wheels with a plurality of annular serrations or cutting edges for use in producing or finishing internal screw threads bya grinding operation.

It is known that if the profile of a grinding Wheel of the kind referred to is formed in the axial plane section as an exact replica of the screw thread it is desired to grind, the resulting thread will in certain cases be found to be inaccurate. The discrepancy is due to the fact that, whereas the thread to be formed is a helix, the operative surface of the grinding wheel consists of a plurality of serrations which are annular. The error is found to increase with the helix angle and is more pronounced in the case of internal grinding of screw threads owing to the increased overlap of grinding surface contact (commonly known as interference) on opposite sides of a plane containing the axes of the grinding wheel and the work.

It follows, therefore, that as nominal true form is normally produced in this plane, any form produced in the work other than in this plane must be to a more or less extent untrue. The increased interference results from the fact that the contacting surfaces of the wheel and work in internal grinding are both curved in the same direction.

The object of the present invention is to adapt the crushing method of forming grinding wheels of the kind referred to to form annular serrations, or cutting edges, of such shape in axial plane section as will produce a screw thread approximating more closely to the correct form than has so far been possible.

In crush-forming or dressing grinding wheels of the kind herein referred to for use in internal form-grinding operations, according to this invention, the improved method consists in giving the annularly serrated crushing roller a bodily movement about a fixed axis and at the same time causing relative axial movement to take place between the grinding wheel and the crushing roller.

The position of the said fixed axis will vary with the core diameter of the internal screw thread to be produced and is in every case such that the distance between said axis and the crests'of the serrations on the roller when engaged with the grinding wheel is preferably equal to the core radius of the internal screw thread to be produced.

The extent of the simultaneous relative axial movement between the grinding wheel and the work will depend upon whether during the grinding operation the axis of the grinding wheel is to be parallel with the axis of the work or inclined thereto to. suit the helix angle of the work. In

the former case the amount of said relative movement must be equal to that proportion of the pitch of the screw thread to be produced which corresponds with the value of the are through which the crushing roller is moved about the fixed axis.

In the accompanying drawings,

Figure 1 is a View in side elevation of a dressing attachment embodying the present invention. Figure 2 is a plan view thereof.

Figure 3 is an end View as seen from the righthand of Figure 1.

Figure 4 is a diagrammatic cross-sectional View illustrating the relative positions of the crushing roller, grinding wheel and work-piece.

The attachment employed for carrying out the improved method of dressing grinding wheels hereinbefore described comprises relatively fixed and movable members of which 2 is mounted on the bed of the machine or upon a slide movable thereon and is formed with an extension 2 carrying guides 2 for the movable member 3 which is capable of a sliding movement in either direction.

Rotatably mounted in the movable member is a shaft 4 one end 4 of which is formed or provided with a lead screw engaging an internally screw-threaded sleeve 5 secured in the fixed member 2 by means of a removable cap 2 and bolts 2 The other end of the shaft 4 is provided with a radial arm 6 fixed thereto, in which an annularly serrated crushing roller 1 is adjustably and rotatably mounted by means of an arbor 8 and a T-headed bolt ii slidably mounted in a T-section slot in the said arm 6. The arrangement is such that when the shaft is rotated the arm is swung about the axis of the shaft and at the same time is also moved axially by the co-action between the lead-screw and the sleeve 5.

The shaft. 4 may be rotated by hand or power. In the construction shown the shaft is fitted with a worm wheel 9 which is engaged by a worm l0 operated by a handle I l.

The grinding wheel I2 is rotatably mounted in a barrel I3 about an axis which is parallel with the axis of the shaft 4 and is adjustable in relation thereto to traverse the grinding wheel across the crushing roller. 1

I l i The position of the roller 1 along the arm 6 is adjusted so that the shortest distance A between the crests of the serrations on the roller and the axis of rotation of the shaft 4 is equal to the core radius B of the internal thread to be produced. The workpiece, represented by the core circle 14 in Figure 4, i arranged substantially co-axial with the axis of the shaft 4, i. e. on the same axis about which the crushing roller swings.

The combined axial and rotary movement of the shaft 4 during the crushing or dressing operation is such that the axial component, and hence the resulting parallel movement of the crushing roller I relatively to the grinding wheel I2 is equal to that portion of the pitch of the thread to be produced corresponding with the value of the arc through which the crushing roller is swung about the axis of the shaft 4.

Instead of providing for axial movement of the shaft 4 relatively to the grinding wheel the latter may be arranged for axial movement during the dressing operation relatively to the shaft and hence to the crushing roller, the extent of such movement, as before, being equal to that portion of the pitch of the thread to be produced corresponding with the value of the are through which the roller is swung.

If required, provision may also be made for tilting the axis of rotation of the crushing roller 5 according to the helix angle of the internal thread to be ground.

I claim:

1. Dressing apparatus for forming the operative surface of grinding wheels used for grinding 2. In a grinding machine the combination with a rotatable grinding wheel, of a dressing attachment therefor comprising a headstock, a member having simultaneous rotary and axial movements therein, an arm fixed to said member, a crushing roller rotatably and adjustably mounted in said arm and means for imparting simultaneous axial and rotary movements to said member whilst the crushing roller is in contact with the grinding wheel.

3. Dressing apparatus for forming the operative surfaces of grinding wheels used for grinding internal screw threads comprising a normally stationary member, a shaft rotatably journalled therein and provided with a threaded portion, an internally threaded stationary sleeve receiving said threaded portion and transmitting axial movement to said shaft during rotary movement thereof, an arm carried by said shaft for rotation therewith, and a crushing roller carried by said arm to swing therewith about the axis of said shaft in operative engagement with a work piece.

4. Dressing apparatus for forming the operative surfaces of grinding wheels comprising, a supporting member, an arm swingably mounted on said member, a crushing roller carried by said arm with its rotational axis parallel to the axis of swinging movement thereof, and means for supporting a grinding wheel adjacent said arm with its rotational axis parallel to the rota tional axis of said crushing roller to have its operative surface engaged thereby during swinging movement of said arm, in combination with mean-s for producing relative axial movement between said crushing roller and grinding wheel simultaneously with and proportional to the swinging movement of said arm.

JOHN ERNEST WAIN WRIGHT. 

